1. Field of Invention
The present invention relates to a liquid crystal apparatus, a driving method thereof, and a projection-type display apparatus and electronic equipment using the liquid crystal apparatus.
2. Description of Related Art
For example, with an active-matrix liquid crystal apparatus, action of writing data to the liquid crystal layer of each pixel is executed by line-at-a-time driving, via switching elements such as a plurality of TFTs (thin-film transistors) connected to a scanning signal line.
Also, in order to eradicate unevenness in the display owing to imbalance in voltage applied to the liquid crystal, and in order to prevent deterioration and so forth of the liquid crystal due to the direct current applied to the liquid crystal, polarity inversion driving is performed, wherein the polarity of the voltage applied to the liquid crystal is inverted at a certain timing.
Polarity inversion driving is a method of driving wherein voltage is applied to one end of the liquid crystal, the polarity (positive or negative polarity) of this voltage is opposite to a reference potential applied to the other end of the liquid crystal. Incidentally, in the present Specification, the term "polarity" refers to the polarity of the voltage applied to both ends of the liquid crystal. In order to perform polarity inversion driving with an active-matrix type device using TFTs, either the potential applied to the common electrode opposing the pixel electrode across from the liquid crystal is changed, or the potential level of the image data signal is changed with reference to the center potential of the voltage amplitude of the image data signal applied to the pixel electrode.
Known types of polarity inversion driving methods involve inversion by the line wherein polarity inversion is performed each time a scanning signal line is selected, or inversion by the line combined with inversion by the dot wherein polarity inversion is performed for each pixel connected to one scanning signal line.
FIG. 9 and FIG. 10 are models for describing the polarity inversion driving method. With conventional active-matrix liquid crystal apparatus, a polarity inversion driving method has been employed wherein line-at-a-time driving is performed and inversion is performed for each pixel (including for each line), and wherein pre-charging of the data signal lines is performed collectively during the blanking period immediately before.
In FIG. 9 and FIG. 10, S1 through S4 represent data signal lines, and H1 through H4 represent scanning signal lines. The "+" and "-" for each pixel represent the voltage applied to the liquid crystal of each pixel, and the polarity of the pre-charge potential supplied to the data signal lines immediately prior to the application of the voltage. FIG. 9 represents the voltage polarity of each pixel at field N, and FIG. 10 represents the voltage polarity of each pixel at field N+1. Regarding polarity inversion driving per pixel and per line, the arrangement is such that differing polarity voltage is applied to each neighboring pixel connected to the same data signal line (each neighboring pixel in the vertical direction in FIG. 9 and FIG. 10).
In this case, even when writing the same black data, for example, on the display to two neighboring pixels which are connected to the same data signal line and connected to different scanning signal lines, the signal level for each of the pieces of black data differs, due to the polarity inversion driving. At this time, since the data signal line itself has parasitic capacity, so time is required for changing the potential of the data signal line from the black level potential on the positive polarity side to the black level potential on the negative polarity side.
With reference to FIG. 11 and FIG. 12, description will be made regarding change in the potential of the data signal line when writing the same black data to two neighboring pixels which are connected to the same data signal line.
In FIG. 11, C10 represents the parasitic capacity of the data signal line S1 (i.e., the equivalent capacity of the data signal line S1). Also, the "-" and "+" noted on the left side of FIG. 11 represents the polarity of the voltage written to the pixels 22 and 24. Incidentally, the pixels 22 and 24 are both to display "black". The pixels are comprised of a storage capacity and a pixel electrode to which data signals are supplied via a switching element, and a liquid crystal layer to which voltage is applied between the pixel electrode and common electrode.
As shown in FIG. 12, during the horizontal scanning time T1, black level potential B1 is applied to one end of the pixel 22 and black is displayed, and during the next horizontal scanning time T2, black level potential B2 is applied to one end of the pixel 24 and black is displayed, in the same manner. In this case, since a common electrode potential which is set between the black levels B1 and B2 is applied to the other end of the pixels 22 and 24, so that voltage of a negative polarity is applied to the pixel 22, and voltage of a positive polarity is applied to the pixel 24, thus inverting the polarity of the voltage applied to the liquid crystal for the same black display. Moreover, with a normally-white display such as described above, the difference in potential between the black level potentials B1 and B2 is the greatest, as compared with display of other gray scales. Accordingly, in the event that pre-charging is not performed, the parasitic capacity C10 of the data signal line S1 must be charged (or discharged) by the image data signal itself, so as to change the potential of the data signal line from the black level potential B1 to B2, as represented by "R1" in the Figure.
Conversely, by performing pre-charging of the same polarity as the polarity of the data signal before supplying the data signal, i.e., by performing pre-charging before the horizontal scanning time T2 so as to maintain the data signal line S1 at the high-voltage second pre-charging potential PV2, as shown as "R2" in the Figure, all that is necessary is to change the potential of the data signal line from the second pre-charging potential PV2 to the black level potential B2, so the amount of charging (discharging) of the parasitic capacity C10 of the data signal line S1 does not have to be great. Accordingly, driving of the liquid crystal is increased in speed.
Now, regarding a conventional liquid crystal apparatus, the arrangement has been such that the black level potentials B1 and B2 are respectively set at 1V and 11V, the white level potentials W1 and W2 are respectively set at 5V and 7V, and the pre-charging potentials PV1 and PV2 are respectively set at 4V and 8V. That is to say, the pre-charge potentials PV1 and PV2 have been set symmetrically to the center potential (6V) between the black level potentials B1 and B2, which are the video amplitude.
The 4V and 8V are voltages which are applied to one end of the liquid crystal via a switching element at the time of displaying intermediate gray scale, and are equivalent to the potential level at the time that the T-V curve, which represents the relation between the voltage applied to the liquid crystal (V) and the transmittance of the liquid crystal apparatus (T), becomes the steepest. In other words, 4V and 8V are equivalent to potential levels at the time that the change in transmittance corresponding to change in voltage applied to the liquid crystal is the greatest. By setting the pre-charging potentials PV1 and PV2 as such, the data signal line can be charged or discharged in a short time from the pre-charging potential to a potential for intermediate gray scale display, so accurate intermediate display can be realized even in the event that the sampling period is reduced.
Now, as described above, image display devices have come to be used for various purposes, such as liquid crystal monitors, notebook-type personal computers, and household equipment. Accordingly, development has been proceeded from the perspective of improving precision and portability thereof. For example, regarding improving precision, development has been proceeded toward a display devices with more pixels, e.g., from VGA (640.times.480 pixels) to XGA (1024.times.768 pixels), from XGA to SXGA (1280.times.1024 pixels), from SXGA to UXGA (1600.times.1200 pixels).
The operating frequencies of the above image display devices differ according to the types of image data signals. For example, VGA is used for monitors for notebook personal computers, and the operating frequencies are 60 Hz, 72 Hz, and 75 Hz. SVGA, for example, is used for monitors for notebook personal computers larger than VGA, and the operating frequencies are 56 Hz, 60 Hz, 72 Hz, and 75 Hz. Further, for example, XGA is used for monitors for desktop personal computers and notebook personal computers, and the operating frequencies are 60 Hz, 70 Hz, and 75 Hz. Also, for example, the operating frequency of EWS (SXGA) is 75 Hz.
For example, in the case that a VGA specifications (60 Hz) device is used for a liquid crystal apparatus, there are 800 dot clock signals in 31778 .mu.sec in one horizontal scanning period, having 640 clocks worth within the effective display period. Accordingly, in the event that the aforementioned driving frequencies of 56, 60, 72, and 75 Hz are applied to this device, the period for each horizontal scanning period is shortened. Also, the image data signals input externally can be compressed or extended by digital processing, thereby performing image display corresponding with each of the image data signals.
Also, such liquid crystal apparatus are applied to projectors and the like, and in this case, the arrangement is such that image display can be carried out by performing compression and expansion of the image data signals appropriately, even in the case that the type of image data signal is switched from one to another.
In according with such increase in the number of pixels in image display devices, the size of liquid crystal panels is increasing, and along with this, irregularities in the image on the image display devices is becoming more recognizable. Against the image irregularities, a measure that is improving the uniformity of the pixels and back-lighting has been taken, thereby reducing irregularities in brightness and color.
However, though various steps are being taken regarding the increased frequency which accompanies the increase in the number of pixels, the switching elements in liquid crystal apparatus are comprised of TFTs. Accordingly, there is the problem that the switching properties are slow not only in data signal sampling but also in pre-charging, and accordingly, study is being made regarding various circuit operations accompanying them.
Also, to the scanning signal line, TFT gates serving as switching elements, the number of which is the equal to the number of pixels in the X-direction, are each connected so the capacity component for the scanning signal line increases. Also, increased panel size means that the wiring resistance of the scanning signals lines increases. Accordingly, the parasitic resistance and parasitic capacity in the scanning signal lines increases and becomes a load, which in turn causes problems of wiring delays.
The present invention has been made in light of the above problems, and it is an object of the present invention to provide a liquid crystal apparatus a driving method thereof, a projection-type display apparatus and electronic equipment using the liquid crystal apparatus, which are capable of preventing deterioration of image quality due to delay of the signal transporting speed when switching, owing to parasitic capacity and parasitic resistance in the supply path of pre-charging signals and parasitic capacity and parasitic resistance in the switching elements.
It is another object of the present invention to provide a liquid crystal apparatus, a driving method thereof, a projection-type display apparatus and electronic equipment using the liquid crystal apparatus, which are capable of preventing deterioration of image quality due to delay of the signal transporting speed when switching, owing to parasitic capacity and parasitic resistance in the scanning signal lines and parasitic capacity and parasitic resistance in the switching elements.
It is a further object of the present invention to provide a liquid crystal apparatus, a driving method thereof, a projection-type display apparatus and electronic equipment using the liquid crystal apparatus, wherein there is no image deterioration even if image data signals of a different type are supplied to the liquid crystal apparatus, by setting the timing for pre-charging and sampling, using the start-up time of the data signal line driving circuit (X-driver) as a reference.